Both holding tank level sensors inop

[NC-Serenity]

A few days out in our 2018 30’ International and have used 50% of our fresh water tank boondocking. For some reason the freshwater level on the panel shows correct at 50% but both Grey and black tanks read 0%. Looked for any leaks on the ground and none.

What common fault would cause both the black and grey tank sensors to stop working at the same time?